Human hair has a variety of textures, from fine to coarse and from straight to very curly. Hair is made of keratin which in turn is made of polypeptide chains bonded together by cystine (or disulfide) bonds, hydrogen bonds and salt linkages. The cortex is the inner part of the hair.
Curly hair is made of hair strands that have irregular surfaces that mesh and tangle with each other to make combing more difficult. Among individuals with excessively curly hair, e.g., individuals of African or Middle Eastern descent, it is especially popular to relax or straighten hair to increase hair manageability and ease of styling. The hair to be relaxed is either pressed or exposed to a relaxer that chemically transforms cystine bonds of the hair to lanthionine bonds. For this reason, the term for the chemical relaxing process is lanthionization.
During chemical relaxing, the hair is wetted with the relaxer and repeatedly smoothed and sometimes partially combed. Typical relaxers contain an alkali metal hydroxide or guanidine hydroxide as an active ingredient. The relaxers greatly decrease the tensile strength of the hair making it more likely to break. Hair being relaxed that is still wet loses as much as 45-50 percent (%) of the hair's original tensile strength. Relaxed hair that is dry loses about 20-25% of the hair's original tensile strength. It is presently theorized that the decrease in tensile strength is due to structural damage associated with a-helical deformation which occurs during relaxing.
The .alpha.-helical deformation of hair can occur under the following conditions:
a) When the high alkalinity relaxers degrade cystine bonds of the .alpha.-keratin during straightening, partial deformation of the .alpha.-helix becomes a reality which causes the loss of tensile strength in the hair fibers. PA1 b) Partial deformation of .alpha.-helix could be caused by the mechanical extension of hair fibers beyond yield and post-yield region which may not be reversible, even when the fibers are allowed to relax, i.e., rest in water. PA1 c) During relaxing, degradation of cystine bonds and low level of mechanical force applied while applying and smoothing relaxer cream on hair causes the deformation of .alpha.-helix in keratine fibers.
In chemical relaxing, once the desired degree of straightening is achieved, the relaxer is rinsed from the hair with water. The hair is then cleansed using a low pH, acidic nondetangling shampoo which is known as a neutralizing shampoo. After shampooing, the hair is very raspy in nature and rough to the touch.
Conditioners are often applied to the hair after shampooing to soften the hair. Quaternary ammonium compounds having chain lengths of about 11 to about 18 carbon atoms are often a component of these conditioners. Unfortunately, most of these compounds do not work when exposed to the relaxer or shampoo, and, therefore, they can only be used in post-shampooing conditioners. For example, U.S. Pat. Nos. 5,060,680 and 5,148,822, both to Akhtar, disclose straightening hair by a method that requires removing substantially all of the relaxer from the hair prior to applying an aqueous hair texturing and strengthening composition to the hair. Furthermore, these compounds do not strengthen hair when incorporated into the relaxer.
It is, therefore, an object of the present invention to provide an improved hair strengthening composition and easy-to-use method for strengthening hair.